pentose phosphate pathway

Question 1

pentose phosphate pathway definition

Answer 1

PPP is a branch of the glycolytic pathway that generates the reducing power NADPH and various different sugars, 3C, 4C, 5C, 6C and 7C

Question 2

what does the PPP separate into?

Answer 2

a irreversible oxidative phase and reversible non-oxidative phase

Question 3

where does PPP take place?

Answer 3

the cytosol

Question 4

is ATP used?

Answer 4

no ATP is directly produced or consumed

Question 5

functions of PPP

Answer 5

synthesis of 5-carbon sugars such as ribose for the formation of DNA and RNA

Biosynthesis of ATP, CoA, NAD, FAD, RNA, DNA

synthesis of NADPH, glutathione antioxidant, lipogenesis

Question 6

stages of the irreversible oxidative reactions

Answer 6

1. glucose/glycogen/pyruvate to glucose 6-phosphate
2. glucose 6-phosphate to 6-phosphogluconolactone, with NADP+ to NADPH catalysed by glucose 6-phosphate dehydrogenase
3. 6-phosphogluconolactone to 6-phosphogluconic acid with the addition of water, catalysed by 6-phosphogluconolactone hydrolase
4. 6-phosphogluconic acid to ribulose 5-phosphate, releasing carbon dioxide and NADPH, catalysed by 6-phosphogluconate dehydrogenase

Question 7

reversible non oxidative interconversions

Answer 7

1. ribulose 5-phosphate to xylulose 5-phosphate catalysed reversibly by phosphopentose epimerase, ribulose 5 phosphate to ribose 5-phosphate catalysed by phosphotenose isomerase
2. xylulose 5-phosphate + ribose 5-phosphate to sedoheptulose 7-phosphate and glyceraldehyde 3-phosphate, catalysed by a transketolase
3. sedoheptulose 7-phosphate and glyceraldehyde 3-phosphate to erythrose 4-phosphate and fructose 6-phosphate catalysed by a transaldolase
4. erythrose 4-phosphate and xylulose 5-phosphate to fructose 6-phosphate and glyceraldehyde 3-phosphate catalysed by a transketolase

Question 8

where do the molecules go?

Answer 8

glyceraldehyde 3-phosphate, fructose 6-phosphate go to glycolysis

ribose- 5-phosphate to nucleic acid biosynthesis

Question 9

transketolase function

Answer 9

moves 2 carbon units, requires thiamine pyrophosphate as a prosthetic group

Question 10

transketolase function

Answer 10

moves 3 carbon units, forms a Schiff base between the enzymes and one of the sugars

Question 11

regulation of PPP explained

Answer 11

oxidative reactions controlled by glucose 6-phosphate dehydrogenase which is stimulated by NADP+ and inhibited by NADPH

non-oxidative regulated depending on availability and need of different sugars

Question 12

what is oxidative stress?

Answer 12

a systemic manifestation of reactive oxygen species and a system’s inability to detoxify. These species include oxide free radicals and peroxides

Question 13

NADPH function explained

Answer 13

an antioxidant

reduces the oxidised GS-SG to two molecules of reduced GSH (glutathione), catalysed by glutathione reductase

hydrogen peroxide becomes converted to two water molecules and activates glutathione peroxidase to form GS-SG again

however as GS-SG is in it the glutathione’s most oxidised form, it will be unable to resist further oxidative stress thus needs to be reduced again by NADPH

Question 14

what do oxide free radical damage?

Answer 14

cells, especially arachidonic acid and linoleic acids

Question 15

illness associated with glucose 6-phosphate dehydrogenase

Answer 15

glucose 6-phosphate dehydrogenase deficiency

Question 16

explain glucose 6-phosphate dehydrogenase

Answer 16

x-linked disorder

most patients are asymptomatic throughout life

G6PD deficiency usually presents as drug induced or infection induced acute haemolytic anaemia

Question 17

when else is the haemolytic seen?

Answer 17

following the indigestion of fava beans- broad beans

Question 18

why does G6PD deficiency lead to haemolytic anaemia?

Answer 18

drugs/infections/fava beans lead to oxidative stress

as sufferers are less able to produce NADPH, they cannot overcome oxidative stress

the red blood cells have no mitochondria, so no other way to produce NADPH and thus are particularly susceptible, leading to their bursting

Question 19

why is G6PD deficiency so common?

Answer 19

thought that the deficiency offers sufferers a level of protection against malaria